Silicone binders for investment casting

ABSTRACT

A green product for use in fabricating a ceramic article comprises a ceramic powder immobilized within a silicone matrix, wherein the silicone matrix comprises one or more cross linked or polymerized silicone monomers and/or oligomers, wherein the one or more cross linked or polymerized silicone monomers and/or oligomers have a alkenyl reactive functional group and a hydride reactive functional group. Processes for forming a green product and a ceramic core with the silicone monomers and/or oligomers are also disclosed.

BACKGROUND

This disclosure generally relates to investment casting, and moreparticularly, relates to silicone-based binders for use in forming theceramic cores and shell molds employed in investment casting.

The manufacture of gas turbine components, such as turbine blades andnozzles, requires that the parts be manufactured with accuratedimensions having tight tolerances. Investment casting is a techniquecommonly employed for manufacturing these parts. The dimensional controlof the casting is closely related to the dimensional control of aceramic insert, known as the core, as well as the mold, also known asthe shell. In this respect, it is important to be able to manufacturethe core and shell to dimensional precision corresponding to thedimensions of the desired metal casting, e.g., turbine blade, nozzle,and the like.

In addition to requiring dimensional precision in the casting of theceramic core, the production of various turbine components requires thatthe core not only be dimensionally precise but also be sufficientlystrong to maintain its shape during the firing, wax encapsulation, andmetal casting processes. In addition, the core must be sufficientlycompliant to prevent mechanical rupture (hot tearing) of the castingduring cooling and solidification. Further, the core materials generallymust be able to withstand temperatures commonly employed for casting ofsuperalloys that are used to manufacture the turbine components, e.g.,temperatures generally in excess of 1,000° C. Finally, the core must beeasily removed following the metal-casting process. The investmentcasting industry typically uses silica or silica-based ceramics due totheir superior leachability in the presence of strong bases.

Investment casting cores made using low pressure casting techniquesgenerally suffer from poor mechanical properties. In low pressurecasting techniques, ceramic slurry containing a solvent and one or morebinders is poured into a mold. Typical binders are sodium silicate,hydrolyzed ethyl silicate, or silica sol as described in U.S. Pat. No.2,928,749. The slurry then “gels” resulting in a rigid solid (sometimesreferred to as a “polymer solvent gel matrix”). The gelled component andsolvent are then removed by heating and/or a combination of heating andsolvent extraction. Poor mechanical properties resulting from the lowpressure casting process causes difficulty in ejecting cast parts fromthe metal die following curing. To impart mechanical strength to thecore, the alcohol solvent commonly employed in the low pressure castingprocess is ignited, bisque firing the part in the casting die prior toejection. This firing step can lead to thermally induced flaws in thecore, reducing its strength and increasing production scrap thatcontinues throughout the metal casting process.

For example, the prior art includes the use of silica (cristobalite) orsilica-zircon as core materials. Dimensional control of the silica coreis difficult for at least two reasons. First, crystalline-based silicamaterials are susceptible to Martensitic-type phase changes during thecasting process. Accordingly, as a practical matter, the degree ofcrystallinity prior to casting is closely controlled. Otherwise, thecore may crack once it is cooled down while still in the associatedmold. Secondly, thermal expansion differences between the silica coreand the associated mold are typically very large. Accordingly, it isdifficult, if not impossible, to tightly fix the silica core within anassociated mold without rendering the silica core susceptible tocracking.

Aluminum oxide, or “alumina”, by itself, without a chemical or physicalbinder material, has also been identified as a potential core material,and is typically employed with reactive alloys. Unfortunately, corescomprised of alumina based ceramics are known to exhibit excessivethermal expansion and poor crush behavior. Such behavior is unacceptablefor applications where dimensional precision is required duringmanufacture, such as in the production of directionally solidified metaleutectic alloys and superalloys, which are typically used formanufacturing of turbine components. Moreover, alumina cores aretypically removed using an autoclave operation, which adds considerableexpense to the process.

Further, shrinkage with a concomitant decrease in porosity results in aceramic article with unsuitable mechanical properties for the casting ofsuperalloys. In this regard, because there generally is a considerablethermal expansion mismatch between the ceramic and the alloy, hoop andlongitudinal tensile stresses are experienced by the alloy upon coolingfrom the superalloy casting temperature. Accordingly, if the ceramicarticle is very dense (i.e., non-porous) with little plasticity andhaving a high resistance to deformation at elevated temperatures, thiscan lead to mechanical rupture or hot tearing of the alloy in theceramic article.

Moreover, with regard to solvents, serious problems can sometimes occur.The various drying procedures available can result in shrinkage andwarping of the article, as capillary forces draw the ceramic particlestogether. Green parts containing high levels of liquids often exhibitthe most shrinkage. Moreover, parts that include both thincross-sections and thicker cross-sections are very susceptible tocracking or distortion, as the thin sections dry faster than the thickersections.

Investment casting molds, or shells, are similar to cores in thatadherence to dimensional tolerances is required for quality castings.Unlike cores, investment casting shells are generally produced vialayer-by-layer application over a pattern such that the shell cavity isdefined by the shape of the pattern. Wax patterns are typically used dueto the ease of fabrication and wax removal. The wax is removed byheating the shell to a temperature above its melting point and pouringout the wax. As a result, traditional manufacturing techniques such asslip casting or injection molding are difficult to implement in shellproduction.

As mentioned earlier, shells are generally produced using alayer-by-layer approach. In this approach, as described in U.S. Pat. No.4,247,333, an alumina-based ceramic with a silica-based binder, similarto those listed for ceramic cores, is applied to the pattern surface,which is then coated with coarse alumina powder. A layer-by-layerprocess is employed to overcome the technical barriers associated withuniformly drying a bulk-ceramic article where the inner surface of thearticle is an impermeable wax interface. By applying relatively thinlayers, drying uniformity is improved, and overall dimensional precisioncan be maintained. Consequently, the shell manufacturing step is arelatively lengthy process; and one in which mold thickness is largelydefined by both composition and number of coatings.

As the designs of alloy castings become more complex, the performance ofthe mold becomes more critical. Consequently, techniques to strengthenthe mold in critical locations have been employed, such as thoseoutlined in U.S. Pat. No. 4,998,581. The need for strengthening arisesfrom the fact that variable shell thickness is difficult to achieveusing conventional layer-by-layer manufacturing techniques.

Accordingly, there remains a need in the art for improved ceramicslurries and more robust processes that provide cores and/or molds withthe desired dimensional accuracy and mechanical properties with minimalshrinkage and warping.

BRIEF SUMMARY

Disclosed herein is a green product for use in fabricating a ceramicarticle, comprising a ceramic powder immobilized within a siliconematrix, wherein the silicone matrix comprises one or more cross linkedor polymerized silicone monomers and/or oligomers, wherein the one ormore cross linked or polymerized silicone monomers and/or oligomers,prior to cross linking and/or polymerization, contain an alkenylreactive functional group and a hydride reactive functional group.

A process for forming a green product comprises mixing a ceramic powderwith silicone monomers and/or oligomers to form a ceramic slurry,wherein the silicone monomers and/or oligomers contain an alkenylfunctionality of formula:

wherein R¹, R², and R³ each independently comprise hydrogen or amonovalent hydrocarbon radical, X a divalent hydrocarbon radical and ais a whole number having a value between 0 and 8, inclusive, and ahydride functionality consisting of silicon-hydrogen bonds; and ahydride functionality consisting of silicon-hydrogen bonds; adding ametallic catalyst compound to the ceramic slurry; and cross linkingand/or polymerizing the silicone monomers and/or oligomers to form arigid silicon matrix.

A process for forming a ceramic core comprises mixing a ceramic powderwith silicone monomers and/or oligomers to form a ceramic slurry,wherein the silicone monomers and/or oligomers comprise an alkenylfunctionality of formula:

wherein R¹, R², and R³ each independently comprise hydrogen or amonovalent hydrocarbon radical, X is a divalent hydrocarbon radical, anda is a whole number having a value between 0 and 8, inclusive, and ahydride functionality consisting of silicon-hydrogen bonds; adding ametallic catalyst to the ceramic slurry; transferring the ceramic slurryinto a mold; cross linking and/or polymerizing the silicone monomersand/or oligomers to form a green product; and heating the green productto a temperature effective to decompose the crosslinked and/orpolymerized silicone monomers and/or oligomers and form a silica char inthe ceramic core.

The above described and other features are exemplified by the followingdetailed description.

DETAILED DESCRIPTION

Disclosed herein are green product compositions and processes suitablefor fabricating cores for use in investment casting. The processgenerally includes dispersing a ceramic powder in a silicone fluid,wherein the silicone fluid includes silicone monomers and/or oligomershaving alkenyl and hydride functionalities. In a preferred embodiment,the silicone monomers and/or oligomers have at least three alkenylfunctionalities and at least three hydride functionalities per monomeror oligomer repeat unit. Once a stable suspension is formed, a metalliccatalyst is added and the desired part is cast. Depending on theparticular monomers/oligomers and metallic catalyst employed, a heatingstep may then be applied to cure the cast suspension into a green body.The silicone monomers and/or oligomers cross link in the mold yielding arigid core of ceramic particles in a silicone based polymeric matrix.The so-formed silicone polymeric matrix may be substantially decomposedin the core or shell to produce a silica char by further heating at ahigher temperature. Advantageously, the compositions and process can beused to provide a reduction in the amount of ceramic powder used in themold, while yielding a core or shell that exhibits minimal volumeshrinkage.

In one embodiment, the silicone monomers and/or oligomers represent thedispersant medium for the ceramic powder, i.e., solvent is not required.As a result, the emission of volatile organic compounds (VOCs) isessentially eliminated. Moreover, since a solvent is not used, specialliquid-permeable molds designed for solvent removal are not needed.Rather, conventional steel molds can be employed representing asignificant commercial and economic advantage. Still further, theabsence of solvent eliminates a drying step that is typically employedin the prior art for removing the solvent, which, as previouslydiscussed, can lead to thermally induced flaws. As a result, cycle timeis reduced since the drying time is essentially eliminated relative tosolvent-based systems, thereby providing additional commercial andeconomic benefits.

In an alternative embodiment, the silicone monomers and/or oligomershaving the alkenyl and hydride functionalities are first dissolved in avolatile solvent, (e.g., aliphatic and aromatic hydrocarbons that can beremoved by heat treatment), which are then added to the ceramic powderto form a ceramic slurry and further processed as generally describedabove. Advantageously, once the solvent is removed, decomposition of thecross-linked silicone binder is relatively easier compared to asolvent-free process due to the interconnected porosity present in thecore as a result of the solvent removal.

Ceramic powders suitable for use in the present disclosure include, butare not intended to be limited to, alumina, fused alumina, fused silica,magnesia, zirconia, spinels, mullite, glass frits, tungsten carbide,silicon carbide, boron nitride, silicon nitride, and mixtures thereof.Most often, the ceramic powder comprises silica, mixtures of silica andzircon, or mixtures of silica and alumina.

Other additives that may be present in the ceramic powder include, butare not intended to be limited to, aluminum, yttrium, hafnium, yttriumaluminate, rare earth aluminates, colloidal silica, magnesium, and/orzirconium for increasing refractory properties of the shell mold or corecomposition. In addition, deflocculants may be added such as stearicacid.

The silicone monomers and/or oligomers having the alkenylfunctionalities that may be added as a binder to the ceramic slurry arealkenyl siloxanes of the general formula (I):

wherein R¹, R², and R³ each independently comprise hydrogen or amonovalent hydrocarbon radical, X a divalent hydrocarbon radical and ais a whole number having a value between 0 and 8, inclusive. The terms“monovalent hydrocarbon radical” and “divalent hydrocarbon radical” asused herein are intended to designate straight chain alkyl, branchedalkyl, aralkyl, cycloalkyl, and bicycloalkyl radicals.

The silicone hydride monomers and/or oligomers are hydrosiloxanes havinghydrogen directly bonded to one or more of the silicon atoms, andtherefore contain a reactive Si—H functional group.

Cross-linking of the silicone monomers and/or oligomers may beaccomplished by utilizing a metal catalyzed reaction of the siliconealkenyl groups and the silicon bonded hydrogen groups. The metalcatalyst, preferably a platinum group metal catalyst, can be selectedfrom such catalysts that are conventional and well known in the art.Suitable metallic catalysts include, but are not intended to be limitedto, the Pt divinylsiloxane complexes as described by Karstedt in U.S.Pat. No. 3,715,334 and U.S. Pat. No. 3,775,452; Pt-octyl alcoholreaction products as taught by Lamoreaux in U.S. Pat. No. 3,220,972; thePt-vinylcyclosiloxane compounds taught by Modic in U.S. Pat. No.3,516,946; and Ashby's Pt-olefin complexes found in U.S. Pat. Nos.4,288,345 and 4,421,903.

Exemplary alkenyl siloxanes useful in the present disclosure includepolyfunctional olefinic substituted siloxanes of the following types:

wherein R is a monovalent hydrocarbon, R′ is an alkenyl radical such asvinyl, or other terminal olefinic group such as allyl, 1-butenyl, andthe like. R″ may include R or R′, a=0 to 20, inclusive, and b=1 to 80,inclusive, wherein a and b are selected to provide a fluid with maximumviscosity of 1,000 centistokes, and such that the ratio of b/a allowsfor at least three reactive olefinic moieties per mole of siloxane offormula (II) above.

Suitable alkyl/alkenyl cyclosiloxanes are of formula (III):[RR′SiO]_(x),  (III)wherein R and R′ are as previously defined, and x is an integer 3 to 18inclusive.

Other suitable functional unsaturated siloxanes may be of the formula(IV):

wherein R, R′, and R″ are as previously defined. Preferably, the ratioof the sum of (c+d+e+g)/f is ≧2.

Exemplary unsaturated siloxanes include1,3-divinyl-tetramethyldisiloxane, hexavinyldisiloxane,1,3-divinyltetraphenyldisiloxane, 1,1,3-trivinyltrimethyldisiloxane,1,3-tetravinyldimethyldisiloxane, and the like. Exemplary cyclic alkyl-or arylvinylsiloxanes include1,3,5-trivinyl-1,3,5-tri-methylcyclotrisiloxane,1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane,1,3-divinyloctaphenylcyclopentasiloxane, and the like.

Suitable polyfunctional hydride siloxanes include compositions depictedbelow:

wherein R is as defined previously, R′″ may include R or H, and a and bare defined as above, and selected such that the ratio of b/a allows forat least three reactive Si—H moieties per mole of siloxane of formula(V) above.

Suitable alkyl/hydride cyclosiloxanes of formula:[HRSiO]_(x),  (VI)wherein R is as previously defined, and x is an integer 3 to 18inclusive.

Other suitable functional hydride siloxanes include:

wherein R and R′″ are as previously defined. Preferably, the ratio ofthe sum of (c+d+e+g)/f is ≧2.

Exemplary silicone hydrides include poly(methylhydrogen)siloxane,poly[(methylhydrogen)-co-(dimethyl)]siloxane;1,3,5,7-tetramethylcyclotetrasiloxane,1,3,5,7,9-decamethylcyclopentasiloxane, and other cyclic methylhydrogensiloxanes; tetrakis(dimethylsiloxy)silane, and organically modifiedresinous hydride functional silicates corresponding to Formula (VII),with the composition [HSi(CH₃)₂O_(1/2)]₂ (SiO₂).

The matrix for the “gel” is selected so as to include at least onealkenyl and hydride siloxane as described above.

Additional terminally functional alkenyl or hydride siloxanes describedbelow in formulas (VIII) and (IX), alone or in combination, may be addedto augment the matrix composition in order to lower the viscosity of theuncrosslinked matrix, effect changes in the cured green body hardnessand strength, and so on, as would be apparent to those skilled in theart in view of the present disclosure.

wherein R and R′ are as previously defined; and n=0 to 100, preferably 0to 30, and most preferably 0 to 10.

It should also be apparent that a satisfactory crosslinked network maybe effected in the course of this disclosure, by combining one componentfrom each of A) a polyfunctional alkenyl or polyfunctional hydridesiloxane, as defined in Formulas (II)-(IV) or Formulas (V)-(VII),respectively; and B) a terminally functional alkenyl or hydride siloxaneas defined in Formulas (VIII) or (IX) respectively, restricted only suchthat the composition contains both an alkenyl and a hydride functionalspecies to allow crosslinking between the complementary alkenyl andhydride reactive functional groups.

The preparation of cores for shell molds and the like generally includesforming a green product from a ceramic slurry comprising the siliconemonomers and/or oligomers, and a ceramic powder using a low pressureforming method, such as gel casting, and then firing the green productup to a temperature of about 900° C. to about 1,650° C. under anoxygen-containing atmosphere. This process facilitates the fabricationof a core with dimensional precision to suit the desired dimensions ofthe superalloy to be cast from such core. Further, cores created fromthis process are dimensionally stable and of a desired strength topermit their deformation during the cooling and solidification of thecasting.

In a preferred embodiment, the ceramic powder is first mixed with one ormore silicone monomers and/or oligomers to form a slurry mixture,wherein the one or more silicone monomers and/or oligomers comprise thealkenyl and hydride reactive functionalities as previously described.The silicone fluid is preferably a liquid at room temperature, and assuch, can be used to provide a low viscosity vehicle for the ceramicpowder. Advantageously, the solvent free ceramic slurry permits the useof conventional molds. That is, molds that are not necessarilyliquid-permeable as is generally required for solvent containing greenproducts. Upon addition of the metallic catalyst, the liquid monomersand/or oligomers can be made to polymerize and/or crosslink to form afirm, strong polymer/solvent (if present) gel matrix. The gel matriximmobilizes the ceramic powder into the desired shape of the mold inwhich the slurry mixture is gelled. The resultant “green” productexhibits exceptionally high strength and toughness (i.e., is notbrittle, resists tearing, cracking, etc.). The alkenyl and hydridefunctional group concentration in the silicone monomers and/or oligomerscan provide a high cross linking density, which results in a high silicayield upon burnout of the molded core. The resultant high silica yieldfrom the matrix allows for the possibility of reduced ceramic powderloadings in the slurry for ease of handling and mold filling whilesimultaneously yielding a part with minimal shrinkage after firing.

The viscosity of the curable silicone matrix, theoretical cross-linkdensity, and resultant silica char yield may be adjusted using theappropriate silicone hydride and alkenyl compounds, and thestoichiometric ratio of total hydride to alkenyl reactive functionalgroups. For instance, the viscosity of the uncured silicone matrix canvary from about 1 to about 1,000 centistokes, preferably about 1 toabout 300 centistokes, and most preferably about 1 to about 100centistokes. The theoretical crosslink density, as defined by theaverage molecular mass of the shortest formula repeat unit distancebetween reactive hydride or alkenyl functional crosslink sites, can varyfrom about 30 to about 4,100 g/mole, preferably from about 30 to about500 g/mole, and most preferably from about 0 to about 150 g/mole. Toproduce a suitably hard and resilient silicone matrix, the hydride toalkenyl ratio is conveniently taken in the range of 0.5 to 3, preferablyin the ratio of 0.5 to 2, and most preferably in the range of 1.0 to1.75. In the particular case of 1,3,5,7-tetramethylcyclotetrasiloxaneand 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane,combinations in molar ratios from 0.5 to 2, gave silica yields uponpyrolysis of the cured matrix at 1,000° C. in air equal to 74 to 87% ofthe original mass.

In another embodiment, the one or more monomers and/or oligomers isfirst dissolved in a solvent prior to mixing with the ceramic powder.Thus, the one or more monomers and/or oligomers solution may be a liquidor solid depending on the physical properties of the monomer. Thesolvent which may be included in the monomer and/or oligomer solutionmay comprise any organic solvent which is a solvent for the monomersand/or oligomers, and preferably exhibits a low vapor pressure at thetemperature at which the one or more monomers and/or oligomerspolymerizes and crosslinks, and exhibits a relatively low viscosity.Preferred solvents include, but are not limited to, aliphatic andaromatic hydrocarbons and other high-boiling point petroleum solvents.

Optionally, the ceramic powder may first be dispersed with a dispersantfor the powder and then mixed with the silicone monomers or oligomers inthe manner previously described, with or without solvent. Variousdispersants for ceramic powders are known in the art and are appropriatefor use in the present disclosure. Care should be exercised however inorder to select a dispersant which does not interact with the monomers,solvent (if present), or metallic catalyst. A particular dispersant maybe evaluated for suitability with a particular ceramic powder and aparticular monomer, solvent and catalyst by mixing small amounts of therespective components and judging the flow properties of the resultantmixture, whether the resultant mixture exhibits a notable yield point,and/or whether the mixture exhibits pseudoplastic behavior. Typicaldispersants include stearic acid, oleic acid, and menhaden fish oil.Generally, the dispersant is used in a small amount, by volume, ascompared with the amount, by volume, of the ceramic powder included inthe mixture.

Generally, the amount of silicone monomer and/or oligomer included inthe ceramic slurry determines the degree of hardness of the resultingsolid, shaped product. Generally, an exceptionally hard green productcan be formed using about 50 to about 75 volume percent of siliconemonomers and/or oligomers having a alkenyl to hydride functional groupratio of about a 1:1 in the green product, and in a preferredembodiment, the one or more silicone monomers and/or oligomers compriseabout 55 to about 65 volume percent, wherein the volume percent is basedon the total volume of the green product.

Once the slurry is prepared, the catalyst is added just prior to thecasting process. Optionally, inhibitors may be added along with thecatalyst to prevent premature gelling before a part is cast. Suchinhibitors are well known in the art, a preferred example of which isprovided in U.S. Pat. No. 4,256,870. Preferably, the slurry mixture withthe catalyst or catalyst/inhibitor is at about room temperature.However, once the slurry mixture is heated, the reaction rate isrelatively high whereby polymerization and cross linking of the siliconemonomers and/or oligomers is easily and quickly achieved. The amount ofmetallic catalyst included in the slurry mixture is generally small ascompared with the amount of monomers and/or oligomers in accordance withconventional curing and cross-linking methods.

The slurry is then transferred, e.g., by extrusion, pouring, syringetransfer, pressing, gravity transfer, and the like, into a closed cavityof the mold. Preferably, where extrusion is used, the ceramic slurry isextruded under low pressure (less than 50 psi) into a die and thengelled. The gelling process is preferably accomplished with heat forrapid manufacturing. However, room temperature gelation is preferredwhere a metal component (if present) reactivity is excessive, e.g.aluminum, wherein the metal component is disposed to react withavailable organic matter to produce undesirable hydrogen gas bubbles.Other molding techniques, including injection molding, may also beemployed. Moreover, any conventional additives known in the ceramicprocessing arts, for example, mold release agents, may be included inthe slurry mixtures for their known functions.

The exact temperature at which polymerization and/or crosslinking occursdepends on the particular metallic catalyst compound and the particularmonomers and/or oligomers that are included. Preferably, the temperatureshould be greater than about room temperature, more preferably at aboutroom temperature to about 120° C., and even more preferably about 50° C.to about 100° C. Similarly, the time necessary to form a firmpolymer-solvent gel matrix is dependent on the particular monomer,solvent, and metallic catalyst compound. Generally, the mold containingthe slurry mixture is preferably heated at an elevated temperature(i.e., greater than room temperature) for at least about five minutes,and more preferably, is heated for a period of about 5 to about 30minutes in order to polymerize and crosslink the monomer and form thefirm polymer-solvent (if present) gel matrix. After polymerization andcrosslinking has occurred, the resultant, solid green body may be cooledto ambient temperature and removed from the mold. If a solvent asdescribed earlier is utilized, the product is in a wet, green conditionin that it may contain solvent and/or is in the unsintered form. Greenproducts thus formed have exhibited extreme strength and toughness.

The wet, green product may subsequently be heated in order tosubstantially remove the solvent and provide a dry product. Removal ofthe solvent, if present, creates interstitial spaces within the ceramic,resulting in an open-pore intermediary product. Such structure speedsdecomposition of the silicone compounds during sintering by promotingmore rapid oxidation, as well as more complete oxidation of the metalliccomponent, e.g., aluminum, if present within green product. This isbecause mass transport of oxygen gas is facilitated by the open porestructure. Although the specific temperature and time necessary fordrying the product depends on the specific metal-containing powder andmonomer solution employed, drying may generally be effected by heatingat a temperature that does not exceed the boiling point of the solvent,if applicable. As such, the temperature is preferably at about roomtemperature to about 100° C., with about room temperature to about 50°C. even more preferred, in an oven for a period greater than about twohours, preferably for a period of from about 2 to about 6 hours. In apreferred embodiment, carried out in the absence of a solvent andwithout need for a solvent-drying step, the well-known thermaldecomposition of the silicone matrix generates low molecular weightcyclic siloxane species, which are driven off and also produces adesirable open-pore intermediary product. Additionally, the siliconepolymer formed may be substantially decomposed to produce a silica charby further heating at a higher temperature, for example, greater thanabout 500° C. Finally, the solid, shaped product may be sintered to forma body of adequate density for use in investment casting. Sinteringtemperatures for various ceramic powders are well known in the art.Alternatively, substantial decomposition of the silicone polymer may beaccomplished as a low temperature step of the sintering process.

In a preferred embodiment, the green product is generally allowed to setin the die for 1 to 2 hours at about room temperature or for about 15minutes at about 50° C.

Once dried, the green product is heated in a conventional kiln under anoxygen-containing atmosphere to a temperature of about 900° C. to about1,650° C. for an aggregate period of about 2 to about 48 hours. Theheating rate is preferably at about 50 to about 200° C./hour.

The following examples are provided to illustrate some embodiments ofthe present disclosure. They are not intended to limit the disclosure inany aspect.

Example 1

In this example, a 38% by volume ceramic article was cast using amixture of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane anda poly[(methylhydrogen)-co-(dimethyl)]siloxane. In this case, the vinylto silicone hydride molar ratio was maintained at 1:1. The ceramic was amixture of amorphous silica and zircon. A 422 gram batch was prepared bymixing the various constituents for 15 seconds in a high-shear mixeroperating at 2,500 rpm. Following mixing, the batch was degassed using arotary vacuum mixer for 5 minutes operating at roughly 50 torr and 60rpm.

The batch was then cast into a mock investment casting core byinfiltrating a specially-designed steel die with the slurry at roughly10 psi pressure. Following casting, the part was cured at 100° C. forroughly 30 minutes prior to part ejection. Following ejection, the partwas cooled to room temperature.

Following casting, the part was fired in air at a rate of 10° C. perhour to a temperature of about 600° C., with about a 5 hour soak at 600°C. The part was then heated to a temperature of about 1,100° C. at arate of 300° C. per hour, with about a 2 hour soak at 1,100° C.Following the soak, the part was cooled to room temperature.

Example 2

Ceramic test bars were cast using a mixture of1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane and1,3,5,7-tetramethylcyclotetrasiloxane in a molar ratio of 1:1. Theceramic was a mixture of amorphous silica and zircon, present at 40% byvolume. A 177.8 gram batch was prepared by mixing the constituents for10 seconds in a high-shear mixer at 2,500 rpm. Following mixing, thebatch was degassed in a rotary vacuum mixer for 5 minutes at roughly 50torr and 60 rpm.

The batch was then loaded into disposable transfer syringes, and handinjected into a custom designed steel casting die. Approximate dieopening dimensions were 0.1″×0.5″×5″. Following casting, the articleswere cured at 50° C. for roughly 90 minutes prior to removal. Theresultant article had a smooth hard surface, was strong and resilient,and could be readily handled without breaking.

Example 3

Ceramic test bars were cast using a mixture of1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane and a hydridefunctional silicate resin corresponding to Formula (VII), containinghydridodimethylsiloxy and silicate units in a molar ratio of 2:1. Thetotal silicon hydride to vinyl molar ratio was 1:1. The ceramic was amixture of amorphous silica and zircon, present at 40% by volume. A177.8 gram batch was prepared by mixing the constituents for 10 secondsin a high-shear mixer at 2,500 rpm. Following mixing, the batch wasdegassed in a rotary vacuum mixer for 5 minutes at roughly 50 torr and60 rpm.

The batch was then loaded into disposable transfer syringes, and handinjected into a custom designed steel casting die. Approximate dieopening dimensions were 0.1″×0.5″×5″. Following casting, the articleswere cured at 50° C. for roughly 90 minutes prior to removal. Theresultant article had a smooth hard surface, was strong and resilient,and could be readily handled without breaking.

The present disclosure provides a number of important advantages byproviding a low pressure method for fabricating a fired ceramic articlefor use as a shell mold or core in the investment casting ofdirectionally solidified eutectic and superalloy materials, which isdimensionally stable and of a desired strength such that it is capableof deformation during cooling and solidification of the casting. In thisrespect, the fired ceramic article is sufficiently porous forfacilitating such deformation. The low shrinkage of the fired ceramicarticle is attributable, at least in part, to the use of highlycrosslinked silicone monomers and/or oligomers having alkenyl andhydride functional groups. Moreover, the use of an in-situ gelationprocess that is free from solvent removes the necessity of igniting thepart after casting. Because of the elimination of the firing step,surface flaws are reduced and an increase in yield rate throughout theinvestment casting process can advantageously be expected. Finally,since a single-step casting technique is employed, variable shellthickness independent of the wax pattern is possible.

While the disclosure has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the disclosure without departing fromthe essential scope thereof. Therefore, it is intended that thedisclosure not be limited to the particular embodiment disclosed as thebest mode contemplated for carrying out this disclosure, but that thedisclosure will include all embodiments falling within the scope of theappended claims.

1. A green product for use in fabricating a ceramic core, comprising aceramic powder immobilized within a silicone matrix, wherein thesilicone matrix comprises one or more cross linked or polymerizedsilicone monomers and/or oligomers wherein the one or more cross linkedor polymerized silicone monomers and/or oligomers, prior to crosslinking and/or polymerization, contain at least three alkenyl reactivefunctional groups or at least three hydride reactive functional groupsper mole of monomer or oligomer; and have a viscosity of about 1 toabout 1,000 centistokes; wherein the green product has the shape of thedesired ceramic core, and wherein the outer surface of the ceramic coredefines the shape of at least one internal cavity of a turbinecomponent.
 2. The green product according to claim 1, wherein thehydride reactive functional groups and the alkenyl reactive functionalgroups are present at a molar ratio of about 0.5 to
 3. 3. The greenproduct of claim 1, wherein the one or more cross linked or polymerizedsilicone monomers and/or oligomers, prior to cross linking and/orpolymerization, are free of solvent.
 4. The green product according toclaim 1, wherein the ceramic powder comprises at least one materialselected from the group consisting of alumina, fused alumina, fusedsilica, magnesia, zirconia, spinels, mullite, glass fits, tungstencarbide, silicon carbide, boron nitride, silicon nitride, andcombinations comprising at least one of the foregoing ceramics.
 5. Thegreen product according to claim 4, wherein the ceramic powder furthercomprises an additive comprising at least one material selected from thegroup consisting of aluminum, yttrium, hafnium, yttrium aluminate, rareearth aluminates, colloidal silica, magnesium, zirconium, orcombinations comprising at least one of the foregoing.
 6. The greenproduct according to claim 1, wherein the one or more silicone monomersand/or oligomers having the hydride functional group is selected fromthe group consisting of: a polyfunctional hydride siloxane of formula:

wherein R is a monovalent hydrocarbon, R′″ is a monovalent hydrocarbonor hydrogen, a=0 to 20, inclusive, and b=1 to 80, inclusive, wherein aand b are selected to provide a fluid with a maximum viscosity of 1,000centistokes, an alkyl/hydride cyclosiloxane of formula:[HRSiO]_(X), wherein x is an integer 3 to 18 inclusive, a functionalhydride siloxane of formula:

wherein a ratio of the sum of (c+d+e+g)/f is 2, a terminal hydridesiloxane of formula:

wherein n=0 to 100, and mixtures thereof.
 7. The green product accordingto claim 1, wherein the one or more silicone monomers and/or oligomerscontaining the alkenyl functional group comprises a formula of:

wherein R¹, R², and R³ each independently comprise hydrogen or amonovalent hydrocarbon radical, X a divalent hydrocarbon radical, and ais 0 or
 1. 8. The green product according to claim 1, wherein the one ormore silicone monomers and/or oligomers having the alkenyl functionalgroup is selected from the group consisting of: polyfunctional siloxanesof formula:

wherein R is a monovalent hydrocarbon, R′ is an alkenyl radical, R″ is amonovalent hydrocarbon or an alkenyl radical, a=0 to 20, inclusive, andb=1 to 80, inclusive, wherein a and b are selected to provide a fluidwith a maximum viscosity of 1,000 centistokes, a cyclic alkyl/alkenylsiloxane of formula:[RR′SiO]_(X), wherein R and R′ are as previously defined; wherein x isan integer 3 to 18 inclusive; an unsaturated siloxane of formula:

wherein R, R′, and R″ are as previously defined; and mixtures thereof.9. The green product according to claim 1, wherein the silicone matrixhas a crosslinik density, as defined by the average molecular mass ofthe shortest formula repeat unit distance between a reactive hydride oran alkenyl functional crosslinik site, of about 30 to about 4,100 gramsper mole.
 10. The green product according to claim 1, wherein the one ormore silicone monomers and/or oligomers having the hydride functionalgroup is selected from the group consisting ofpoly(methylhydrogen)siloxane,poly[(methylhydrogen)-co-(dimethyl)]siloxane;1,3,5,7-tetramethylcyclotetrasiloxane,1,3,5,7,9-decamethylcyclopentasiloxane, cyclic methylhydrogen siloxanes;tetrakis(dimethylsiloxy)silane, hydridodimethylsiloxy silicate[HSi(CH₃)₂O_(1/2)]₂ (SiO₂), and mixtures thereof.
 11. The green productaccording to claim 1, wherein the one or more silicone monomers and/oroligomers having the alkenyl functional group is selected from the groupconsisting of 1,3-divinyl-tetramethyldisiloxane, hexavinyldisiloxane,1,3-divinyltetraphenyldisiloxane, 1,1,3-trivinyltrimethyldisiloxane,1,3-tetravinyldimethyldisiloxane,1,3,5-trivinyl-1,3,5-tri-methylcyclotrisiloxane,1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane,1,3-divinyloctaphenylcyclopentasiloxane, and mixtures thereof.
 12. Thegreen product according to claim 1, further comprising at least onesolvent.
 13. An investment casting core fabricated with the greenproduct of claim 1.